feat(ctf): per-instance flags derived via HMAC(CTF_KEY, slug) #316

Context

All 36 flags are static and published in prisma/flags.ts, so anyone with repo access can submit them without solving anything — making "CTF" inaccurate for a competition where players see the source.
Decide the design before inviting a PR: this touches the seed, verify route, walkthroughs (which quote literal flags), the Hall of Fame, ...

Design decisions to make first

• Flag format when keyed (keep OSS{…}? length?).
• Training mode (no key) must stay identical to today for self-study.
• What to do about walkthroughs that quote literal flag values.

Tasks (after the note)

• Add CTF_KEY; when set, derive each flag as HMAC(CTF_KEY, slug) at seed/verify time.
• Keep unkeyed training mode unchanged.
• Update /api/flags/verify and the seed; document both modes.

Acceptance criteria

• With CTF_KEY set, flags differ per deployment and no repo literal validates; training mode unchanged.

Files

• prisma/flags.ts, prisma/seed.ts, app/api/flags/verify/route.ts, docs